1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a driving circuit which solve the problem of deterioration in a display quality, thereby realizing a uniform display quality in an active matrix type liquid crystal display device used in a variety of office automation devices including personal computers and word processors, multimedia information terminals, audio vidual devices, game machines, and the like.
2. Description of the Related Art
In recent years, due to the advent of the highly-informationalized society, demand for a display capable of displaying a large amount of information at once has been rapidly increased. Conventionally, a CRT (cathode ray tube) has been generally used for displaying a large amount of information. However, the CRT is generally large in size and consumes a large amount of power. Since the CRT is made as a stationary type device, the CRT is not suitable for use as a portable device. On the other hand, a flat display such as a liquid crystal display device is thin, and is light in weight. Such characteristics of the flat display are attracting attention.
Liquid crystal display devices are roughly classified into two groups, i.e., a passive matrix type and an active matrix type. A super twisted nematic (hereinafter, referred to as "STN") liquid crystal display device which is a typical passive matrix type liquid crystal display device and a thin film transistor (hereinafter, referred to as "TFT") liquid crystal display device which is a typical active matrix type liquid crystal display device will be described hereinafter.
The TFT liquid crystal display device includes switching elements such as TFTs which are positioned at intersections of row electrodes and column electrodes arranged in an matrix. Display is performed by controlling the switching element so as to independently apply a voltage to a liquid crystal layer in each of pixels. In such a TFT liquid crystal display device, liquid crystals are operated in a TN mode. Thus, it is possible to realize both a high contrast and high-speed response.
According to the STN liquid crystal display device, on the other hand, a liquid crystal layer is interposed between a pair of glass substrates on which row electrodes and column electrodes are provided so as to cross each other. A display is realized by changing optical state of the liquid crystal layer depending on an RMS voltage level of a driving voltage which is applied between the row electrodes and the column electrodes.
If these two types of the liquid crystal display devices are compared with each other in terms of their costs, the STN liquid crystal display device is superior to the TFT liquid crystal display device due to its simple panel structure and fabrication process.
As to their display performances, however, the TFT liquid crystal display device has an advantage over the STN liquid crystal display device which has no switching element associated with a pixel. In particular, the STN liquid crystal display device tends to have a deteriorated display quality as its display capacity increases. This is because as its display capacity increases, its driving margin reduces, thereby reducing its contrast ratio, and display unevenness which depends on its display pattern, i.e., cross-talk, occurs.
Regarding their optical response performances, the optical response speed of the STN liquid crystal display device is generally about 300 ms, whereas that of the TFT liquid crystal display device is about 50 ms. Therefore, the STN liquid crystal display device has an optical response speed slower than that of the TFT liquid crystal display device, and thus is not suitable for displaying a moving picture. Moreover, in the STN liquid crystal display device, its contrast ratio tends to reduce as its response speed increases.
As described above, both types of the liquid crystal display devices have their advantages and disadvantages. Along with an increase in the use of multimedia, however, even the relatively inexpensive STN liquid crystal display device came to be required to display a moving picture (e.g., a video picture, a picture, or the like). Needs for a high-speed responsiveness and a high picture quality are increasing.
Hereinafter, the cause of a reduction in its contrast in the STN liquid crystal display device having a high-speed responsiveness and a technique for improving the problem will be described.
The STN liquid crystal display device conventionally employs a line-sequential driving method. This driving method sequentially scans a group of row electrodes one line at a time during one frame period. Upon scanning, a high level scanning pulse is applied to each of the row electrodes only once in the one frame period. Synchronizing with the application of the high level scanning pulse, a data voltage which complies with display data in each pixel related to the scanned row electrode is applied to a column electrode.
It is intended that the liquid crystal display device employing the conventional line-sequential driving method mainly displays a still picture or the like. Such a liquid crystal display device conventionally uses a liquid crystal material having a relatively low response speed. In such a case, the liquid crystal molecules respond to an applied RMS (root-mean-square) voltage (i.e., effective voltage), thereby obtaining a practical contrast ratio. However, if high speed responsiveness of the liquid crystal layer is realized by reducing the liquid crystal viscosity or reducing a thickness of the liquid crystal layer in order to realize the display of a moving picture, the liquid crystal molecules respond not to the RMS voltage (i.e., effective voltage) but to a driving waveform itself according to the line-sequential driving method. As a result, the phenomenon in which a transmittance varies for each of the frames becomes prominent. This phenomenon is referred to as a "frame response phenomenon". The frame response phenomenon leads to a significant reduction in the contrast ratio.
In order to improve such a problem, unlike the line-sequential driving method in which the high level scanning pulse is applied only once in the one-frame period, a driving method in which the high level scanning pulse is divided into a plurality of low level scanning pulses and applied in the plurality of times in one frame, thereby suppressing the frame response phenomenon to prevent the reduction in the contrast ratio has been suggested. Such a driving method is referred to as a multiline selection driving method. Such a driving method is characterized in that a plurality of row electrodes are simultaneously scanned using an orthogonal matrix. Hereinafter, the fundamental operation thereof will be briefly described.
After performing an orthogonal transformation operation for input image data by using the orthogonal matrix, a data voltage based on its arithmetic data is applied to a column electrode. In synchronization with the application of the data voltage, a scanning voltage based on a column vector of the orthogonal matrix is applied to all of the simultaneously-selected row electrodes at the same time. In this manner, the orthogonal inverse transformation of an image data is performed on the liquid crystal panel. As a result, the input image can be reproduced. Depending on the number of the row electrodes which are simultaneously selected, their scanning order, or the like, the following three driving methods have been suggested. However, the fundamental principals thereof are as described above.
The first driving method is an active addressing method in which all of row electrodes for an entire display screen are simultaneously scanned. This method is disclosed in T. J. Scheffer et al. (SID '92, Digest, p. 228), Publication for Opposition No. 7-120147, and the like.
The second driving method is a sequency addressing method in which a plurality of row electrodes which are fewer than the total number of row electrodes for an entire display screen are grouped, and the resulting groups are scanned in a sequential manner. The sequency addressing method can have a smaller circuit as compared to the active addressing method. This second driving method is disclosed in T. N. Ruckmongathan et al. (Japan Display '92, Digest, p. 65), Japanese Laid-open Publication No. 5-46127, and the like.
According to the third driving method disclosed, for example, in Japanese Laid-open Publication No. 6-291848, a screen is divided into a plurality of blocks along the row direction; a plurality of row electrodes which are fewer than the total number of row electrodes in each of the blocks are grouped; and the resulting groups are sequentially scanned so as to drive all of the blocks. This third driving method can further reduce a memory capacitance as compared to the second driving method. Therefore, the third driving method can realize a circuit smaller than that employed by the second driving method.
As described above, by employing the multiline selection driving method for the passive matrix type liquid crystal display device having a high-speed responsiveness, it becomes possible to suppress the frame response phenomenon and improve the reduction of the contrast ratio.
Next, the cross-talk, which depends on a display pattern, will be described in the case where the liquid crystal display device employs the multiline selection driving method as an example.
FIG. 6 schematically illustrates a liquid crystal display device 100' employing the conventional multiline selection driving method. As shown in FIG. 6, the liquid crystal display device 100' includes a timing control circuit 1', a frame memory 2', an orthogonal matrix generator 3, an orthogonal transformation circuit 4, a group 7 of row drivers, a group 8U of column drivers for the upper section of a screen, a group 8L of column drivers for the lower section of the screen, and a liquid crystal panel 9. The liquid crystal panel 9 has row electrodes 91 (the number of the row electrodes 91 is 2.times.N) and M column electrodes 92 which are arranged so as to cross the row electrodes 91, where N=(1, 2, . . . , n), and M=(1, 2, . . . , n). The intersections of the row electrodes 91 and the column electrodes 92 are arranged in a matrix. A liquid crystal layer (not shown) is interposed between the row electrodes 91 and the column electrodes 92, and each intersection of the row electrode 91 and the column electrode 92 respectively corresponds to each pixel. The liquid crystal layer in each of the pixels changes its optical state depending on an RMS voltage level of a driving voltage which is applied between the row electrodes 91 and the column electrodes 92. In this manner, display is performed.
The passive matrix type liquid crystal display device has a tendency that an operation margin represented by the following Expression 1 reduces as the number of the row electrodes increases (i.e., as N increases), thereby reducing the passive matrix type liquid crystal display's contrast ratio. ##EQU1##
Accordingly, upon performing mass display, a dual-scan type liquid crystal panel wherein a screen is divided into two sections as shown in FIG. 6 and each of the two sections is independently driven are generally used. Although the case where the upper section of the screen is driven will be described hereinafter, the same process is performed in order to drive the lower section of the screen.
Display data S101 is input to the frame memory 2' in a single scanning manner. Specifically, the display data S101 is written to the frame memory 2' for every row. Since the liquid crystal display device 100' employs the multiline selection driving method, display data written to the frame memory 2' is read out as follows. L.times.M (rows.times.columns) display data S201 corresponding to L row electrodes 91 which are simultaneously selected in N.times.M display data for a screen (i.e., the upper section of the screen) is read out for every column, and then output to the orthogonal transformation circuit 4. In this manner, display data is written along the row direction, and read out along the column direction according to the multiline selection driving method. The orthogonal matrix generator 3 generates an orthogonal matrix, and outputs a column vector S301 of the generated orthogonal matrix to the orthogonal transformation circuit 4 and the group 7 of row drivers so as to make the matrix correspond to display data S201 which is read out from the frame memory 2'.
The orthogonal transformation circuit 4 receives data S201 which is output from the frame memory 2'. By using column vector S301 of an orthogonal matrix, which corresponds to data S201, orthogonal transformation operation is performed. Its arithmetic data S401 is output to the group 8U of column drivers for the upper section of the screen.
Based on the column vector S301 of the orthogonal matrix which is output from the orthogonal matrix generator 3, the group 7 of row drivers applies a scanning voltage, which is enough for L electrodes, to the row electrodes 91 of the liquid crystal panel 9 such that the scanning voltage corresponds to arithmetic data S401. Similarly, the group 8U of column drivers for the upper section of the screen applies a data voltage to the column electrodes 92 of the liquid crystal panel 9 based on arithmetic data S401, which is output from the orthogonal transformation circuit 4.
As shown in FIG. 6, the liquid crystal panel 9 is a dual-scan type liquid crystal panel wherein the panel is divided into two sections, i.e., the upper section of the panel and the lower section of the panel, and the two sections are driven independent of each other. N row electrodes are provided for each of the upper and lower sections of the panel 9. The group 7 of row drivers consists of a plurality of row drivers 7-1, 7-2, . . . , 7-Y depending on the number of the row electrodes 91, i.e., N. Based on the column vector S301 of the orthogonal matrix which is output from the orthogonal matrix generator 3, the group 7 of row drivers sequentially applies a scanning voltage, which is enough for simultaneously-selected L row electrodes, to the row electrodes 91. Similarly, the group 8U of column drivers for the upper section of the screen includes a plurality of column drivers 8U-1, 8U-2, . . . , 8U-X depending on the number of the column electrodes 92, i.e., M, and applies a data voltage based on arithmetic data S401 which is output from the orthogonal transformation circuit 4 to M column electrodes 92 simultaneously. As a result, the orthogonal inverse transformation of display data is performed on the liquid crystal panel 9, thereby realizing the display of data.
The timing control circuit 1' controls a timing for the entire system of the liquid crystal display device 100'.
Each of driving circuits in the thus-structured liquid crystal display device 100' employing the multiline selection driving method will be described taking the case where the number of simultaneously-selected row electrodes is set to four as an example.
FIGS. 7A and 7B are timing charts showing control for the operation of the frame memory 2'. FIG. 7A is a diagram for describing the writing operation to the frame memory 2'. FIG. 7B is a diagram for describing the reading operation from the frame memory 2'. In FIGS. 7A and 7B, a "Vsync signal" refers to a vertical synchronous signal, and a "Hsync signal" refers to a horizontal synchronous signal. Both of these signals are input with display data S101. One cycle of the Vsync signal is referred to as "a vertical scanning period", and one cycle of the Hsync signal is referred to as "a horizontal scanning period".
As shown in FIG. 7A, in the case where display data for 2.times.N rows is input, an Enable signal, indicating the period during which display data is effective, maintains a level H only during consecutive 2.times.N horizontal scanning periods in a vertical scanning period. Display data is input to the frame memory 2' in a single scanning manner according to the Enable signal, and written to the frame memory 2'. Herein, input data from 1 to N is display data for the upper section of the screen, and input data from N+1 to 2.times.N is display data for the lower section of the screen. In a vertical scanning period, a horizontal scanning period in which the Enable signal is not effective is called a "vertical interval". A plurality of consecutive horizontal scanning periods are generally included in a vertical interval.
FIG. 7B is a diagram for describing the reading operation of display data for the upper section of the screen from the frame memory 2'. Since the liquid crystal panel 9 employed in this example is a dual-scan type panel wherein the upper section of the panel and the lower section of the panel are simultaneously driven, display data input in a single scanning manner is read out twice in one vertical scanning period if the reading operation is processed in the same clock frequency as that of the writing operation. Hereinafter, a reading period of display data read out at a time is referred to as "one frame period". For an every frame period, display data of four rows which are simultaneously selected is read out from the frame memory 2' four times, and output to the orthogonal transformation circuit 4. It is required that the reading period has 2.times.N=((N rows/4 rows).times.4 times.times.2 frames) horizontal scanning periods in one vertical period. FIG. 8 shows an exemplary structure of the orthogonal transformation circuit 4. d0 to d3 shown in FIG. 8 represent four rows of display data S201 which is read out from the frame memory 2'. In the case of two-gray-scale display (i.e., black-and-white display), each of d0 to d3 is represented by one bit of "0" or "1". The set of f0 to f3 is a column vector S301 of the orthogonal matrix which is output from the orthogonal matrix generator 3, and each of f0 to f3 is represented by one bit of "0" or "1". Then, the orthogonal transformation operation represented by the following Expression 2 is performed to derive its arithmetic values G (g0, g1, g2). ##EQU2##
Table 1 shows the relationship among an arithmetic value G, arithmetic data S401, and an output data voltage. As shown in Table 1, an arithmetic value G is an integer from 0 to 4. Therefore, as shown in FIG. 8, the arithmetic values G are output as 3-bit arithmetic data S401 from g0 to g2 to the group 8U of column drivers for the upper section of the screen. A data voltage corresponding to arithmetic data S401 is applied to the column electrodes 92 of the liquid crystal panel 9 via the group 8U of column drivers for the upper section of the screen.
TABLE 1 Arithmetic data Output data Arithmetic value (upper.rarw. .fwdarw.lower) voltage 0 000 -2Vc 1 001 -Vc 2 010 0 3 011 +Vc 4 100 +2Vc
For reference, exemplary truth value tables for full adder and half adder are shown below.
 Full adder Input Carry Sum A B I C S 0 0 0 0 0 0 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 0 0 0 1 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1
 Half adder Input Carry Sum A B C S 0 0 0 0 0 1 0 1 1 0 0 1 1 1 1 0
FIG. 9 is a view for describing how cross-talk, which depends on a display pattern, occurs on the liquid crystal panel 9 in the liquid crystal display device 100' employing the multiline selection driving method, which is driven in the manner as described above.
FIG. 9 shows display states of the liquid crystal panel 9. Herein, pixels indicated by white color are in a lighting state, and pixels indicated by black color are in a non-lighting state. Areas shown by oblique lines indicate pixels which fail to be in the bright state due to a reduced transmittance caused by cross-talk. Reference numeral Y1 denotes any one of the group of row electrodes, and two column electrodes which cross the row electrode Y1 are denoted by X1 and X2. Pixels positioned at the intersections of the row electrode Y1 and the column electrodes X1 and X2 are denoted by P1 and P2, respectively. For the convenience of illustration, distortion in the waveform of a scanning voltage at pixel P1 and that in pixel P2 are supposed to be the same.
Each of FIGS. 10A and 10B shows exemplary waveforms of data voltages which are applied to the column electrodes X1 and X2 in FIG. 9. FIG. 10A shows actual voltage waveforms, and FIG. 10B shows ideal voltage waveforms. The "ideal voltage waveform" as used herein refers to a predetermined voltage waveform desirable to apply across a liquid crystal layer in a pixel region. Since the pixel region has a capacitance and resistance, the waveform of a voltage actually applied across the liquid crystal layer (shown in FIG. 10A) is different from the predetermined voltage waveform desirable to apply across a liquid crystal layer shown in FIG. 10B.
As is apparent from the ideal voltage waveforms shown in FIG. 10B, RMS voltages which are identical with each other in the ideal state are respectively applied across pixels P1 and P2 associated with the common row electrode Y1. Therefore, no difference should occur in the transmittances of the liquid crystal panel at pixels P1 and P2. In reality, however, due to the resistance component of the electrode or the capacitance component of the liquid crystal layer, distorted waveforms as shown in FIG. 10A are applied across the pixels of the liquid crystal panel.
As described above, due to the difference in display patterns, differences occur among distortion levels in waveforms of data voltages which are respectively applied across column electrodes. As a result, as shown in FIG. 9, even when the same bright state is achieved at both of pixels P1 and P2, pixel P1 associated with the column electrode X1, which has a large distortion level in its data voltage waveform, has a reduced transmittance as compared to pixel P2 associated with the column electrode X2, which has a smaller distortion level in its data voltage waveform. Due to the reduced transmittance, the display becomes dark. In this manner, cross-talk occurs in pixel P1. Since the cross-talk which depends on a display pattern significantly deteriorates the display quality, cross-talk is a very important problem to be solved in the passive matrix type liquid crystal display device.
Accordingly, in order to overcome the problem of such cross-talk, the following two techniques have been suggested for the line-sequential driving method.
According to the first technique, a period during which a data voltage waveform is inverted is provided every predetermined horizontal scanning period. As a result, even when waveform distortion due to its display pattern does not exist, the waveform of the data voltage is made distorted, thereby uniformalizing the distortion levels of the waveform to some extent. This technique is disclosed in Japanese Laid-open Publication Nos. 5-333315 and 4-276794, and the like.
The second technique is a method in which a compensation voltage corresponding to a reduced amount of an RMS voltage accompanied by waveform distortion for every column electrode is applied depending on the number of changes in the polarity of the data voltage. This technique is disclosed in Japanese Laid-open Publication No. 3-210525 and Japanese Patent Application No. 7-98825, and the like.
However, the aforementioned techniques for solving the cross-talk which depend on a display pattern have problems as described below.
According to the aforementioned first technique, transmittance of the entire liquid crystal panel is reduced, thereby reducing its contrast ratio.
Moreover, data voltage waveforms, which are applied to column electrodes in a background display region where most of all the column electrodes in the entire liquid crystal panel are provided, simultaneously change in all scanning periods. As a result, a large level of waveform distortion is generated in the row electrodes via the capacitance of the liquid crystal layer. Consequently, cross-talk which is different from the aforementioned cross-talk in kind is increased. This kind of cross-talk becomes more prominent as a potential change in the column electrodes increases. Thus, the first technique is not suitable for the multiline selection driving method wherein a data voltage is higher than that in the line-sequential driving method.
Furthermore, the first technique causes a waving phenomenon wherein a scanning line looks as if it is flowing for every cycle in which the data voltage waveform is inverted. Although there is only one scanning line according to the line-sequential driving method, a plurality of scanning lines exist according to the multiline selection driving method. Therefore, the waving phenomenon becomes more prominent in the case where the multiline selection driving method is employed. Although the waving phenomenon is less apparent in the liquid crystal panel having relatively slow responsiveness, the waving phenomenon appears to be more prominent in the liquid crystal panel having high-speed responsiveness. This is because the tendency of the liquid crystal to respond to the scanning pulse intensifies if the data voltage waveform is inverted in the same cycle as that in the liquid crystal panel having a relatively slow responsiveness. Regarding such a point, the multiline selection driving method also has a disadvantage over the line-sequential driving method.
According to the aforementioned second technique, the number of possible values a data voltage can take is increased in the multiline selection driving method as compared to the line-sequential driving method.
According to the line-sequential driving method, a data voltage takes only two values, i.e., the data voltage for on-display and the data voltage for off-display. Therefore, it is possible to count the number of changes in the polarity of a data voltage. Thus, the result of such a counting has a one-to-one correspondence with the display data.
According to the multiline selection driving method, on the other hand, a data voltage can take many values (i.e., the number of selected lines+1). For example, in the case where four row electrodes are simultaneously driven, five values are needed as shown in Table 1. Since the case where a data voltage has no polarity, i.e., the case where the data voltage is 0, is included among the five values, the second technique in which changes in polarity are counted cannot be applied to the multiline selection driving method.
Specifically, in the line-sequential driving method, a data voltage takes only two values with respect to the non-selection level of a scanning signal, i.e., +V or -V. Therefore, in both of the cases where the data voltage changes from +V to -V and where the data voltage changes form -V to +V, the amounts of waveform distortion (i.e., the required amounts of compensation) are the same. In other words, the total amount of required compensation is proportional to the number of changes in the polarity of the waveform. On the other hand, in the multiline (2 or more lines) selection driving method, the possible levels of a data voltage are three or more (i.e., the number of selected lines+1). In the case where four lines are simultaneously selected, for example, a data voltage takes five values with respect to the non-selection level of the scanning signal, i.e., -2V, -V, 0, +V, and +2V. In such a case, the case where a data voltage level changes from -2V to +V and the case where a data voltage level changes from -2V to +2V, for example, are the same in terms of their polarity change, i.e., a change from a negative value to a positive value. However, amounts of their required compensation are different from each other. In the case where a data voltage level changes from +V to +2V, its polarity does not change. However, since its waveform changes, compensation in an amount corresponding to the waveform distortion is necessary. (In addition, in the case where a data voltage level changes from 0, which has no polarity, to a positive or negative value, or in the case where a data voltage level changes from a positive or negative value to 0, which has no polarity, it is necessary to determine as to whether the polarity is changed or not changed.) As described above, according to the multiline selection driving method, the required compensation amount is not necessarily proportional to the number of changes in the polarity of a data voltage. As a result, simply applying a compensation voltage corresponding to the number of changes in its polarity is not appropriate compensation.
According to a method in which compensation is performed by newly providing a compensation voltage, since a data voltage takes two values in the case of the line-sequential driving method, when a compensation voltage is newly provided in each of the data voltage levels, for example, the data voltage takes four values in total. Therefore, compensation can be relatively easily performed. However, in the case where the same processing is performed in the multiline selection driving method, since a data voltage takes many values, it is difficult to newly provide compensation voltages.
As described above, in the case where the multiline selection driving method is employed for the passive matrix type liquid crystal display device having high-speed responsiveness, it is extremely difficult to employ techniques which have been applied for the conventional line-sequential driving method. Therefore, it is impossible to solve the problem of cross-talk sufficiently.